This invention relates to electromagnetic axial thrust bearings and, particularly, to thrust bearings with high force modulating capacity.
Electromagnetic axial thrust bearings are known and disclosed, for example, in U.S. Pat. Nos. 4,180,296 and 4,353,602. Electromagnetic thrust bearings comprise at least one electromagnet for counteracting the thrust, a displacement detector and a feedback control circuit for controlling the excitation of the electromagnet in response to changes in displacement. The axial thrust electromagnetic bearing typically comprises a stationary member or stator including an exciting winding and a low reluctance path for magnetic flux. A low reluctance rotating member or rotor is arranged in close proximity to the stationary member so that flux lines pass between the stationary and rotating members with a large component of the magnetic lines in the axial direction.
As is well known with transformers, motors, and electromagnetic bearings, rapidly changing flux induces eddy currents which in turn oppose change in flux. In the case of an electromagnetic bearing, opposition to change in flux increases the response time of the thrust bearing magnetic circuit. Laminated cores are known for reducing eddy currents. However, application of a laminated core to the rotor of an electromagnetic thrust bearing is not shown in the prior art. As correctly stated in U.S. Pat. No. 4,180,296, rotors cannot be made of laminated material because the mechanical strength of the rotor would be too weak.
The '296 patent discloses a thrust bearing having a large number of radial semicircular slots to reduce eddy currents. The disadvantages of this structure is that the slots themselves being filled with air or a nonconducting material have low magnetic permeability and reduce the total flux carrying capacity of the rotor. The thickness of the material remaining between the slots is difficult to reduce as much as might be desired. If the slots are not filled, the rotor becomes an annoying siren.
U.S. Pat. No. 4,353,602 discloses a magnetic bearing in which the problem of eddy currents in the rotor due to axial thrust magnetic bearing simply is not addressed. If the force in the electromagnetic thrust bearing is to be rapidly modulated, the eddy currents in both the stator and the rotor must be controlled.
It is known in the art to construct thrust bearing stators of magnetic cores comprising radially disposed E-shaped laminates. The laminates are stacked into milled radial slots. In this structure, sectors of the stator are not laminated since tapered laminates are not practical. In this case, the rotor is simply a solid piece of steel.